The present invention relates to a continuous form printer using a continuous form sheet.
In general, a continuous form printer includes a tractor unit having a pair of tractor belts and pulleys for driving the tractor belts. Each tractor belt is provided with protrusions for engaging with feed holes formed at a predetermined pitch at the sides of the continuous form sheet.
In order to, detect a leading edge of the continuous form sheet after the continuous form sheet is set in the tractor unit or after the printer is turned on, a top sensor is provided downstream of the tractor unit in the feeding direction of the continuous form sheet. A conventional top sensor includes a swingable lever swingable between an erect position where the swingable lever projects across the feeding path of the continuous form sheet, and a retracted position where the swingable lever does not project into the feeding path. When a continuous form sheet is present at the position of the swingable lever, the swingable lever is pressed down by the continuous form sheet below the feeding path. The top sensor further includes a detecting sensor for detecting the position of the swingable lever. In general, the detecting sensor is arranged to generate an OFF signal when the swingable lever is in the erect position and to generate an ON signal when the swingable lever is in the retracted position. The passage of the leading edge of the continuous form sheet can be detected in response to a change in the signal from the detecting sensor.
However, there is a possibility that the continuous form sheet may bend or be warped, causing a portion of the continuous form sheet to be deflected upward. In this case, if the portion of the continuous form sheet deflected upward is above the swingable lever of the top sensor, the swingable lever may not be pressed down by the continuous form sheet even though the continuous form sheet, is present. Consequently, the detecting sensor may incorrectly send an OFF signal even though the continuous form sheet is correctly positioned. In such a case, a control unit of the printer may mistakenly determine a leading edge of the continuous form sheet, which causes a deviation of the printing position on the continuous form sheet.
More particularly, a recently proposed printer is arranged to discharge a printed page of the continuous form sheet out of the printer so that a user may check the result of the printing or may separate a page from the continuous form sheet. In such a printer, since a user touches the discharged sheet for checking or separating, the chance of the continuous form sheet being bent (or warped) and deflected is increased. Thus, the above-mentioned incorrect detection of the leading edge and resulting incorrect printing may occur more often.